This invention relates to an electric fuse of the current-limiting type and, more particularly, to a fuse of this type which can interrupt a wide range of abnormal currents but is especially adapted to interrupt relatively low overcurrents.
It is well recognized that a type of abnormal current that is difficult for a current-limiting fuse to interrupt is a relatively low overcurrent that persists for a long period of time before melting the fusible element or elements of the fuse. For example, in a typical current-limiting fuse, an overload current of 1.5 to 3 times the steady-state rating of the fuse may persist for an hour or even more before it melts the fusible elements of the fuse.
To assist in extinguishing the arc or arcs formed upon such melting, it has been common to provide, in each region where an arc is anticipated, structure of gas-evolving material which evolves an arc-extinguishing gas when exposed to the arc. Examples of such fuses are disclosed in U.S. Pat. Nos. 3,766,509-Cameron; 3,238,333-Kozacka; 4,167,723-Wilks, 3,437,971-Mikulecky, and 3,562,162-Pitha.
In studying typical fuses of this general type, we have observed that during a prolonged period of low overcurrent such as referred to above and prior to arc-initiation, the gas-evolving structure evolves substantial amounts of gases in response to heating of the fuse elements. Such gas evolution is undesirable for at least the following reasons:
1. The total available amount of easily-evolved gas is limited, and any amount lost before arc-initiation is not available for arc-extinction. Insufficient gas flow at the time of arc-initiation could interfere with arc-extinction.
2. Gas evolved before arcing may cause excessive pressure to develop in the fuse.
3. The gas release process is endothermic so that the desired temperature rise of the fusible element or elements can be retarded by such gas evolution. This may distort the time-to-melt v. current curve of the fuse by increasing the melting time at low overcurrents.